Recently, an image display device such as a liquid crystal or an organic EL has been developed considerably, and the usage thereof has been expanded from a medium or small image display device such as a mobile phone or a personal computer monitor to a large image display device for a television. In such an image display device, in order to improve image display performance, various phase difference films are used for homogeneity of the display, improvement of contrast, enlargement of the viewing angle of the display, and the like. In order to obtain brilliant colors and a high-definition image, it is required that the phase difference film has uniform birefringence over the entire surface, and that optical properties are not changed even under environments of severe temperature and humidity conditions.
As a resin material to be used, a transparent film formed of polymer such as a polycarbonate (PC) or a cyclic olefin polymer is generally used. In general, in a case of a phase difference film which is formed by stretching and orienting a film formed of a polycarbonate, a photoelastic constant increases to approximately 100×10−12/Pa, and thus birefringence excessively increases and becomes uneven, and the birefringence is changed by slight stress which is generated at the time of assembling such as laminating or an environmental change. In addition, except for some brands of the phase difference film, wavelength dependency of a phase difference increases in a positive sign (a phase difference at a short wavelength increases with respect to the center wavelength and a phase difference at a long wavelength decreases with respect to the center wavelength), and thus light leakage occurs, and the homogeneity and the contrast of the display are degraded.
In order to solve the problems generated from the original physical properties of the resin, a phase difference film which is formed by stretching and orienting a film formed of a cyclic olefin polymer which is a material having a comparatively small photoelastic constant is used (for example, Patent Document 1 and Patent Document 2). Among them, it is disclosed that a phase difference film which is prepared by stretching and orienting a film formed of a cyclic olefin polymer having a photoelastic constant in a range of 0 to 20×10−12/Pa has excellent initial properties, is rarely affected by a usage environment or manufacturing conditions, and has excellent reliability (Patent Document 1).
In addition, the wavelength dependency of the phase difference is comparatively small, and thus the phase difference film formed of the cyclic olefin polymer is preferably used as a display material member. However, even when the phase difference film formed of the cyclic olefin polymer of the related art is a material having an excellent optical function such as transparency and a phase difference, adhesiveness at the time of laminating the film or at the time of mounting the film on the image display device is not sufficient. Further in the present in which high-definition, energy saving, or the like of the image display are further required, there are demands that the performance of a phase difference function such as wavelength dependency is further improved, and that the transparency of the film contributing to the energy saving is increased.
The properties of the wavelength dependency of the phase difference are able to be divided into three categories of properties having the positive sign described above, properties not having wavelength dependency, and properties having a negative sign (the phase difference at the short wavelength decreases with respect to the center wavelength, and the phase difference at the long wavelength increases with respect to the center wavelength), and thus the phase difference film is suitably arranged and used in various image devices by using the respective properties.
Here, in particular, a wavelength dispersion of a negative sign is generally referred to as reverse wavelength dispersion. The materials of the related art are mostly materials having a positive sign, and in particular, in an initial liquid crystal display material using a material which has a comparatively large positive sign, the material caused a decrease in the performance of the display. Studies have been made thereafter to form a polymer material of the film into a composition, or to make the material multi-layered, and thus it has become possible to comparatively decrease positive wavelength dependency. However, sufficient performance has not been obtained in some usage.
In addition, as a polymer material not having wavelength dependency, polyvinyl alcohol (PVA) is known. However, the polyvinyl alcohol does not have sufficient dimensional stability in a usage environment and requires a protective film such as seen in a polarization plate, and thus the film formed of the material is not able to be practically used as a phase difference film. Further, as the material having the negative sign, a phase difference film obtained by stretching a film formed of a special polycarbonate is used in some applications (Patent Document 3). However, problems may occur during usage due to a photoelastic constant specific to the polycarbonate resin described above. In addition, as the material having the negative sign, a system using a composition of the polymer material (Patent Document 4), a system laminating the phase difference film (Patent Document 5), a system using a special cyclic olefin polymer in a film material (Patent Document 6 and Patent Document 7), and the like are disclosed, but no system is widely used from a viewpoint of the complexity of each step such as a synthesizing step of the polymer material, a film preparing step, and a stretching step, or the price of the product, and the like.
Further, as a fluorine-containing cyclic olefin copolymer, a copolymer having a 5-membered ring structure (A) and a plurality of cyclic structures (B) in a main chain is included, and it is known that an optical material having a molar ratio (A)/(B) of 95/5 to 1/99 has low birefringence even when a polymer chain itself is oriented (Patent Document 8). Even when this copolymer is stretched, it is not possible to manufacture the phase difference film.